Unfortunately, despite the apparent merits of utilizing linear-scaling PBE0-TS [1]}
in studying liquid water and ions, the prefactor of the linear-scaling method is still large and the computational costs are extremely high.
In this regard, the community awaits an accurate yet efficient XC functional that can be utilized to study
not only liquid water but also \(\rm OH^{-}\) (aq) and \(\rm H_{3}O^{+}\) (aq) ions.
Notably, the recently proposed SCAN functional, [2]}
fulfilling all 17 known exact requirements on the semilocal XC functional, can achieve similar accuracy as PBE0-TS does in describing the structural and dynamic properties of liquid water at ambient conditions, as well as the water density.[3]}
In particular, the computational cost of SCAN is comparable to GGA functionals but about an order of magnitude smaller than the PBE0-TS functional for studying condensed phases.[4]}
Compared to GGA functionals,
SCAN predicts stronger covalent bonds within water molecules and less negatively charged local environment around the oxygen atom.
As a result, the directional HBs are weakened.
In addition, the intermediate-ranged van der Waals interaction in SCAN provides attractive forces
among water molecules, which leads to the movements of the second-shell water molecules towards the
non-H-bonded interstitial area, creating a more disordered and denser structure
on the intermediate range.
The above changes provided by SCAN results in a weakened H-bond network of liquid water that is closer to the experimental data. [5]}, [6]}
As a result, SCAN provides better structural and dynamic properties of liquid water than PBE. [3]}
This is considered to be the main reason why SCAN,
rather than PBE and other GGA functionals, predicts the correct quantitative relation
between densities of water and ice under ambient conditions. [3]}
Furthermore, SCAN also exhibits high precision in modeling water clusters, gas and ice phase water,
which outperforms GGA functionals. [9]}, [10]}, [11]}, [12]}, [13]}
